The principal objectives of this project are: 1. To exploit genetic mutants of Neurospora crassa in determining the relationships between membrane fatty acid composition, phospholipid composition, sterol composition and membrane physiology. 2. To generate excitable membrane systems with altered membrane lipid composition. 3. To utilize excitable membrane systems with altered lipid composition to explore the relationship between membrane excitation and lipid composition. By utilizing and generating organisms with genetic defects in lipid metabolism, by manipulating the lipid content of growth media and diet, and by altering environmental temperature, organisms with altered membrane lipid composition will be generated. Differential scanning calorimetry will be used to determine the relationship between membrane lipid composition and the gel-liquid crystal phase transition. The importance of this transition as well as the importance of individual lipid components to membrane function will be assessed by electrophysiological techniques. Intracellular microelectrodes will be used to monitor in vivo membrane resistance and membrane potential as indices of passive and active transport processes, respectively. Extracellular techniques will monitor conduction velocity, refractory period, and duration of the action potential of excitable membranes to determine the role of lipids in controlling the excitation process.